Wireless communication system

ABSTRACT

A method for communication includes transmitting a first uplink message from a first remote node ( 200, 300, 400 ) to a central node ( 100 ) in a wireless communication system according to a first frequency hopping scheme, and transmitting a second uplink message from a second remote node to the central node in the wireless communication system according to a second frequency hopping scheme, different from the first scheme. Both the first and the second uplink messages are received and processed at the central node.

CROSS REFERENCE TO RELATED APPLICATION

The present invention claims from the benefit of U.S. ProvisionalApplication No. 60/927,506, filed May 2, 2007, which is incorporatedherein by reference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to wireless communication systems. Somewireless communication systems, such as event reporting systems, usesynchronized, time-slotted, frequency-hopping schemes and switchedantenna diversity mechanisms

SUMMARY OF THE INVENTION

There is provided, in accordance with an embodiment of the presentinvention, a method for communication, including:

-   -   transmitting a first uplink message from a first remote node to        a central node in a wireless communication system according to a        first frequency hopping scheme;    -   transmitting a second uplink message from a second remote node        to the central node in the wireless communication system        according to a second frequency hopping scheme, different from        the first scheme; and    -   receiving and processing both the first and the second uplink        messages at the central node.

In a disclosed embodiment, the second frequency hopping scheme is anon-synchronized frequency hopping scheme, and receiving the seconduplink message includes scanning a receiver of the central node over thefrequencies used in the second frequency hopping scheme in order todetect the second uplink message. The second frequency hopping schememay be used by remote nodes in the wireless communication system thatare not synchronized with the central node.

In some embodiments, the first frequency hopping scheme is asynchronized, time-slotted, frequency hopping scheme. The method mayinclude synchronizing the first remote node with the central node priorto transmission of the first uplink message. A second remote node maytransmit the second uplink message using the second frequency hoppingscheme in order to join the wireless communication system and becomesynchronized with the central node.

Additionally or alternatively, the second remote node transmits thesecond uplink message using the second frequency hopping scheme in orderto re-synchronize with the central node after having lostsynchronization or in order to deliver an operational message to thecentral node after having lost synchronization. The second remote nodemay transmit the second uplink message using the second frequencyhopping scheme in order to re-synchronize with the central node afterhaving lost synchronization in addition to delivering the operationalmessage.

Optionally, the second frequency hopping scheme is used by remote nodesin the wireless communication system that include a one-way radiotransmitter for delivering operational messages.

Typically, receiving and processing both the first and the second uplinkmessages includes implementing both the synchronized and thenon-synchronized frequency hopping schemes together in a receiver of thecentral node. In some embodiments, implementing both the schemesincludes defining a time-slot that includes a system frequency window(SFW) for receiving the first uplink message and a scanning window (SCW)for receiving the second uplink message. Receiving the first uplinkmessage may include tuning the receiver of the central node during theSFW to a current frequency value of a system frequency-hopping function,and upon receiving a valid preamble, remaining tuned to the currentfrequency value until the entire first uplink message has been received.Additionally or alternatively, receiving the second uplink messageincludes performing a fast frequency scan during the SCW, and upondetecting a valid preamble at a given frequency, remaining tuned to thegiven frequency until the entire second uplink message has beenreceived.

In some embodiments the method may also include defining an antennaswitching function of the central node, the function specifyingrespective time slots in which each of a plurality of antennas of thecentral node is to receive signals,

-   -   wherein transmitting the first uplink message includes        selecting, at the first remote node, an antenna of the central        node as a favored antenna for receiving transmissions from the        first remote node at the central node, and transmitting the        first uplink message in a time-slot selected responsively to the        antenna switching function so that the message will be received        by the favored antenna at the central node.

Additionally or alternatively, the method may include selecting, foreach first antenna of the central node, a respective favored secondantenna among two or more second antennas of the first remote node fortransmitting signals that will be received at the central node via thefirst antenna, and transmitting the first uplink message in a specifiedtime slot via the favored second antenna with respect to the firstantenna specified by the antenna switching function for the specifiedtime slot.

In some embodiments, transmitting the first uplink message includesforwarding an uplink message received by the first remote node which isconfigured to operate as a repeater.

In a disclosed embodiment, receiving and processing the uplink messagesincludes monitoring events detected by the remote nodes, and the methodincludes issuing an alarm in response to one or more of the detectedevents.

There is also provided, in accordance with an embodiment of the presentinvention, a wireless communication system, including:

-   -   a plurality of remote nodes, which are configured to operate in        accordance with first and second different frequency hopping        schemes; and    -   a central node, which is configured to receive and process both        a first uplink message transmitted by a first remote node in the        wireless communication system according to the first frequency        hopping scheme and a second uplink message transmitted by a        second remote node in the wireless communication system        according to the second frequency hopping scheme.

There is additionally provided, in accordance with an embodiment of thepresent invention, apparatus for wireless communication, including:

-   -   a radio transceiver, which is configured to transmit uplink        messages to a central node in a wireless communication system;        and    -   a processor, which is coupled to drive the radio transceiver to        transmit a first uplink message to the central node according to        a first frequency hopping scheme and to transmit a second uplink        message to the central node according to a second frequency        hopping scheme, which is different from the first frequency        hopping scheme.

There is further provided, in accordance with an embodiment of thepresent invention, apparatus for wireless communication, including:

-   -   a radio transceiver, which is configured to receive uplink        messages transmitted by remote nodes in a wireless communication        system in accordance with first and second different frequency        hopping schemes; and    -   a processor, which is coupled to process both a first uplink        message transmitted by a first remote node in the wireless        communication system according to the first frequency hopping        scheme and a second uplink message transmitted by a second        remote node in the wireless communication system according to        the second frequency hopping scheme.

There is moreover provided, in accordance with an embodiment of thepresent invention, a method for communication, including:

-   -   transmitting periodic status messages from a remote node to a        central node in a wireless communication system;    -   in response to the periodic status messages, transmitting        acknowledgment messages from the central node to the remote        node, the acknowledgement messages including time-stamps; and    -   synchronizing the remote node with the central node using the        time-stamps.

In a disclosed embodiment, synchronizing the remote node includesaligning a value of a local clock at the remote node with a system clockmaintained by the central node. Additionally or alternatively,synchronizing the remote node includes aligning a frequency of the localclock with an actual frequency of the system clock using thetime-stamps. In one embodiment, the periodic status messages aretransmitted with a fixed interval between the status messages, andadjusting the frequency includes computing a frequency adjustment usingthe time-stamps and the fixed interval without performing a divisionoperation.

There is additionally provided, in accordance with an embodiment of thepresent invention, a wireless communication system, including:

-   -   a remote node, which is configured to transmit periodic status        messages; and    -   a central node, which is configured to transmit acknowledgment        messages to the remote node in response to the periodic status        messages, the acknowledgement messages including time-stamps,    -   wherein the remote node is configured to synchronize with the        central node using the time-stamps.

There is furthermore provided, in accordance with an embodiment of thepresent invention, a method for communication, including:

-   -   transmitting downlink messages in a time-division-duplexing        (TDD) wireless communication system using a first frequency        hopping function; and    -   transmitting uplink messages in the TDD wireless communication        system using a second frequency hopping function, which is        synchronized with but different from the first frequency hopping        function.

The first and second frequency hopping functions may be mutuallyorthogonal or mutually pseudo-orthogonal.

Typically, transmitting the downlink messages includes transmitting thedownlink messages from a central node in the wireless communicationsystem during predetermined downlink windows, and the method includesreceiving the downlink messages at receivers of remote nodes in thewireless communication system, wherein the remote nodes actuate thereceivers only during the downlink windows. In a disclosed embodiment,when the central node does not have any outstanding downlink messagesfor transmission, the uplink messages are received at the central nodeduring the downlink windows. Receiving the downlink messages at theremote nodes may have priority over transmitting the uplink messages.

In one embodiment, transmitting the downlink messages includestransmitting at least some of the downlink messages from the centralnode to one or more repeaters in the wireless communication system in amanner identical to transmitting the downlink messages from the centralnode to the remote nodes. Additionally or alternatively, transmittingthe downlink messages includes transmitting at least some of thedownlink messages from the central node to one or more repeaters in thewireless communication system in a manner identical to transmitting ofthe uplink messages from the remote nodes to the central node, and themethod may include receiving the at least some of the downlink messagesfrom the central node at the repeater in a manner identical to receivingthe downlink messages from the central node at a remote node.Alternatively, the method includes receiving the at least some of thedownlink messages from the central node at the repeater in a manneridentical to receiving the uplink messages from the remote nodes at therepeater.

There is also provided, in accordance with an embodiment of the presentinvention, a wireless communication system, including:

-   -   a central node, which is configured to transmit downlink        messages in a time-division-duplexing (TDD) wireless        communication scheme using a first frequency hopping function;        and    -   one or more remote nodes, which are configured to receive the        downlink messages and to transmit uplink messages to the central        node using a second frequency hopping function, which is        synchronized with but different from the first frequency hopping        function.

There is also provided, in accordance with an embodiment of the presentinvention, a method for communication, including:

-   -   defining an antenna switching function of a first node in a        wireless communication system, the function specifying        respective time slots in which each of a plurality of antennas        of the first node is to receive signals;    -   selecting, at a second node in the wireless communication        system, an antenna of the first node as a favored antenna for        receiving transmissions from the second node; and    -   transmitting a message from the second node to the first node in        a time slot selected responsively to the antenna switching        function so that the message will be received by the favored        antenna at the first node.

Typically, selecting the antenna includes evaluating respective scoresof the antennas of the first node.

In some embodiments, evaluating the respective scores includesevaluating a history of successful receptions via each of the antennas.

Additionally or alternatively, evaluating the respective scores includesevaluating a quality of a respective propagation channel between thesecond node and each of the antennas.

In some embodiments, evaluating the quality of the respectivepropagation channel includes evaluating signal quality parametersmeasured by the second node when receiving an acknowledgment transmittedvia each of the antennas.

Additionally or alternatively, evaluating the quality of the respectivepropagation channel includes measuring signal quality parameters at thefirst node when receiving transmissions from the second node, andincorporating a value of the measured signal quality parameters withinthe ACK reply for use in computing the respective scores at the secondnode.

Commonly, the signal quality parameters evaluated at the first nodeand/or the second node include a received signal level.

There is also provided, in accordance with an embodiment of the presentinvention, a method for communication, including:

-   -   defining an antenna switching function of a first node in a        wireless communication system, the first node having a plurality        of first antennas, the function specifying respective time slots        in which each of the first antennas is to receive signals;    -   selecting, for each first antenna among the plurality of the        first antennas, a respective favored second antenna among two or        more second antennas of a second node in the wireless        communication system for transmitting signals that will be        received via the first antenna; and    -   transmitting a message from the second node to the first node in        a specified time slot via the favored second antenna with        respect to the first antenna specified by the antenna switching        function for the specified time slot.

In some embodiments, transmitting the message includes forwarding anuplink message.

There is also provided, in accordance with an embodiment of the presentinvention, a wireless communication system, including:

-   -   a first node, including a plurality of antennas and having a        predefined antenna switching function, which specifies        respective time slots in which each of the plurality of the        antennas is to receive signals; and    -   a second node, which is configured to select one of the antennas        of the first node as a favored antenna for receiving        transmissions from the second node, and to transmit a message to        the first node in a time slot selected responsively to the        antenna switching function so that the message will be received        by the favored antenna at the first node.

There is also provided, in accordance with an embodiment of the presentinvention, a method for communication, including:

-   -   a first node, including a plurality of first antennas and having        a predefined antenna switching function, which specifies        respective time slots in which each of the first antennas is to        receive signals; and    -   a second node, which includes two or more second antennas, and        which is configured to select, for each first antenna among the        plurality of the first antennas, a respective favored second        antenna among the two or more second antennas for transmitting        signals that will be received via the first antenna, and to        transmit a message to the first node in a specified time slot        via the favored second antenna with respect to the first antenna        specified by the antenna switching function for the specified        time slot.

In some embodiments, the second node includes a repeater, which isconfigured to forward the message from a remote node.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

FIG. 1 presents a block diagram of a wireless event monitoring system,in accordance with an embodiment of the present invention;

FIG. 2 presents an example of a time-slotted scheme, in accordance withan embodiment of the present invention;

FIGS. 3( a) and 3(b) are timing diagrams that illustrate a power-savescheme, in accordance with an embodiment of the present invention;

FIG. 4 presents a block diagram of a node in a wireless communicationsystem, in accordance with an embodiment of the present invention;

FIG. 5 is a timing diagram that illustrate active synchronization withreceiver fast frequency scan, in accordance with an embodiment of thepresent invention; and

FIGS. 6( a), 6(b) and 6(c) are timing diagrams that illustratesconcurrently employment of access schemes, in accordance with anembodiment of the present invention.

FIG. 7 is a timing diagram that illustrates an intra-message switchedantenna diversity scheme, in accordance with an embodiment of thepresent invention;

FIG. 8 a timing diagram that presents an example of an antenna switchingfunction, in accordance with an embodiment of the present invention;

DETAILED DESCRIPTION OF EMBODIMENTS

The embodiments that are described herein below relate to wirelesscommunication systems, and more specifically to synchronized,time-slotted, frequency-hopping wireless communication systems. Suchsystems commonly require methods for initial synchronization,maintenance of synchronization, re-synchronization in cases ofsynchronization loss, high reliability of message delivery, low latencyof message delivery, message delivery during periods of synchronizationloss, antenna diversity, low current consumption of battery-powerednodes, low cost and simple implementation. An example of a synchronized,time-slotted, frequency-hopping wireless communication system in whichthese embodiments could be applied is a wireless event reporting system,which serves in the description that follows as a platform fordescribing the features of these embodiments and the manner in whichthey meet the above requirements. This event reporting system isdescribed solely by way of example, and the principles of the presentinvention may similarly be applied in wireless communication systems ofother types.

FIG. 1 presents a block diagram of a wireless event reporting system,for example a wireless alarm system, in accordance with an embodiment ofthe present invention. The system comprises a central unit 100 and oneor more distributed monitoring devices 200. The system might alsocomprise one or more signaling devices 300, one or more human interfacedevices 400 and one or more repeaters 500. Throughout the presentdocument, the components of the system are also referred to as nodes,the central unit and the repeaters are referred to as central nodes andall other nodes are referred to as remote nodes.

The monitoring devices detect events of interest and report them to thecentral unit. The monitoring devices might, for example, be any of thevarious detectors of an alarm system, such as motion detectors, glassbreak detectors, magnetic contact detectors, smoke/fire detects, gasdetectors, flood detects, panic buttons, human health monitors andsimilar devices.

The signaling devices produce signals according to commands receivedfrom the central unit. For example, a signaling device might be an audiosignaling device such as a siren or a visual signaling device such as awarning light. The signaling device might also be a communication devicefor sending messages from the event monitoring system to other systems,for example a device that sends messages to a remote location over thetelephone line.

The human interface devices provide the system user with a remoteinterface to the central unit. For example, a human interface devicemight be a remote keypad.

Communication between the nodes of the wireless event monitoring systemis usually based on a TDD (time division duplexing) scheme, meaning thatat a given moment the radio transceiver is able either to transmit or toreceive. The TDD scheme is selected since it is more economical than thealternative FDD (frequency division duplexing) scheme, but the methodsdescribed herein below may alternatively be adapted for use in FDDsystems.

Communication between the nodes of the wireless event monitoring systemcomprises uplink messages, transmitted from the remote nodes to thecontrol unit, directly or via one or more repeaters, and downlinkmessages, transmitted from the control unit to the remote nodes,directly or via one or more repeaters. Both types of messages areusually relatively short. For example, the length of a typical messagetransmitted in an alarm system is typically on the order of ten to fiftybytes of data.

Uplink traffic comprises event-reporting messages and might alsocomprise periodic status messages. An event-reporting message is sentfrom a monitoring device to report an event detected by the device.Usually, event-reporting messages are infrequent. For example, in analarm system, the average rate of event reporting messages is typicallyon the order of ten to fifty messages per day. On the other hand, thosemessages need to be delivered with high reliability and at shortlatency. For example, typical requirements in the context of an alarmsystem are probability of missed events less than about 10⁻⁵ and latencyon the order of about one or two seconds.

Status messages are sent periodically from a remote node to indicate thestatus of the sending node. Periodic reception of status messages from aremote node also provides an indication of the quality of the wirelesslink from the remote node to the control unit or the repeater. Forexample, the European standard EN 50131-5-3 for wireless alarm systemsspecifies that in a grade 3 system the status RF link should bemonitored at least once per 100 seconds, which calls for periodic statusmessages at intervals of less than 100 seconds between successivemessages.

Downlink traffic comprises messages transmitted from the control unit tothe signaling devices and the human interfaces devices, and might alsocomprise messages transmitted from the control unit to the monitoringdevices. Messages to the signaling devices are usually infrequent andneed to be delivered with high reliability and short latency. Typicalrequirements in the context of alarm systems are similar to therequirements for event reporting messages.

Communication between the control unit and a given remote node might beeither direct, or via a repeater or a sequence of repeaters, dependingon system deployment and on the quality of the wireless links betweenthe remote node and the central unit and the between the remote node andthe repeaters.

The central nodes of an event monitoring system are typically mainspowered, while the remote nodes are typically battery powered. Batterysize and battery life-time are usually important factors in wirelessevent monitoring systems, and therefore one of the importantconsiderations in devising a communication scheme for a wireless eventmonitoring system is to minimize the current consumption of the remotenodes.

Since a primary function of the distributed monitoring devices is tosend report messages, those devices might in principle be fitted withone-way radio transmitters rather than with two-way radio transceivers.Throughout the present document, devices fitted with one-way radiotransmitters are referred to as one-way nodes and devices fitted withtwo-way radio transceivers are referred to as two-way nodes. Althoughone-way nodes might be more economical, two-way nodes enable much betterperformances in various aspects, some of which are described below. Anembodiment of the present invention that is described hereinbelowrelates to systems in which all the remote nodes are two-way nodes. Theprinciples of the present invention, however, may also be applied insystems in which some of the monitoring devices are one-way nodes.

One of the advantages of fitting the remote nodes of the wireless eventmonitoring system with a two-way radio transceiver is the ability toutilize an automatic repeat request (ARQ) mechanism for the uplinktraffic. According to the ARQ mechanism, when an uplink messagetransmitted by a given remote node is successfully received by a centralnode, the central node replies with an acknowledgement (ACK) message. Ifthe remote node does not receive an ACK, the remote node retransmits themessage one or more times, until an ACK is received, or until some limitfor the number of retransmissions is reached. An alternative mechanism,in case ARQ cannot be utilized, is blind repetition, wherein everymessage is transmitted several times. The advantages of ARQ over blindrepetition are increased reliability and reduced current consumption.Another advantage of ARQ over blind repetition is the lower occupancy ofthe wireless media, which implies lower probability of collision.

Another advantage of two-way communication is the ability to maintainsynchronization. Synchronization means that every node in the wirelessnetwork is fitted with a local clock. The local clock of the centralunit is referred to as the system clock, and all other nodes keep theirlocal clocks synchronized with the system clock. The mechanism ofsynchronization is based on time stamps transmitted by the central nodesand received by the other nodes. Although wireless event monitoringsystem might, in principle, be non-synchronized, synchronization enablesmuch better performances in various aspects, some of which are describedbelow.

One of the advantages of synchronizing a wireless event monitoringsystem is the ability to employ a time-slotted access scheme for theuplink traffic. According to the time-slotted access scheme, the timeaxis is divided into time-slots, and the duration of a time-slot issufficient to accommodate a typical uplink message following by an ACKreply. The advantage of the slotted access scheme is reduced probabilityof collision, which implies higher reliability, lower latency and lowercurrent consumption.

An example of a time-slotted scheme for the uplink of a wireless alarmsystem is presented in FIG. 2. According to this example, the timeaccess is divided into super-frames of fixed duration, for example 100seconds, each super frame is divided into frames of fixed duration, forexample one second, and each frame is divided into time-slots of fixedduration, for example 40 ms. Each remote node in the system is assigneda respective frame, which restricts the number of remote nodes in thisspecific example to 100. The first time-slot, or the first fewtime-slots, in each given frame, are dedicated for transmission ofuplink periodic status messages from the remote node associated with thegiven frame, and the rest of the time-slots within each frame are usedfor transmission of uplink random access messages, such as eventreporting messages. This scheme eliminates potential collision of agiven periodic status message with other periodic status messages,because each node transmits the periodic status messages in a differentframe. This scheme also eliminates potential collision of a givenperiodic status message with random access messages, because randomaccess messages are not transmitted in the first time-slot, ortime-slots, of the frame that are dedicated for periodic statusmessages. The probability of collision between random access messages islow, due to the low frequency of those events, and in case suchcollision does occur, it is recovered by the ARQ mechanism, with randomback-off.

Another advantage of synchronizing a wireless event monitoring system isthe ability to employ a power save scheme for the downlink. According tothe power save scheme, the receiver of the remote node spends most ofthe time in sleep mode, also referred to as stand-by mode or power-savemode. When in sleep mode, most of the functions of the receiver aredisabled, thus keeping the current consumption as low as possible. Thelocal clock of the remote node is retained operative in sleep mode too,in order to maintain synchronization with the system. The local clock isalso utilized to wake up the receiver for the downlink windows, asdescribed below.

According to the power save scheme, downlink messages are transmitted atdetermined points in time, referred to as downlink windows. It isadvantageous, although not necessary, to align the downlink windowstructure with the uplink time-slot structure. A power-save scheme,aligned with the uplink time-slot structure, is presented in FIGS. 3( a)and 3(b), wherein FIG. 3( a) illustrates the case of no downlinktransmission and FIG. 3( b) illustrates the case of a downlinktransmission. The downlink window, denoted in the Figures as DW, islocated at the beginning of the time-slot.

According to the power save scheme, the receiver of the remote node iswoken up at the beginning of the downlink window and is kept open for ashort time-period, referred to as the snapshot window, trying to detecta valid preamble transmitted by a central node. The snapshot window isdenoted in the Figures as SNPW. If a valid preamble is detected duringthe snapshot window, the receiver remains open, detects the rest of themessage and then returns to sleep mode, as illustrated in FIG. 3( b).The preamble is denoted in the Figure as PR. Otherwise, the receiverreturns to sleep mode after the snapshot window, as illustrated in FIG.3( a).

The interval between downlink windows might depend on the node type. Forexample, a typical arrangement for a wireless alarm system would be toschedule downlink windows for the signaling devices on the order of onceper second, while scheduling downlink windows for monitoring devices onthe order of once per minute.

An advantage of the power-save mode for a wireless event monitoringsystem is the great reduction in current consumption. A limitation ofthe power save mode is that a downlink message can be repeated, eitheraccording to an ARQ scheme, or according to a blind repetition scheme,only at the next downlink window. Therefore, when power-save mode isemployed, it is desirable to protect the downlink messages, as much aspossible, from being interfered with by other messages. A method forproviding such protection is described further hereinbelow.

A block diagram of a node in a wireless communication system, forexample a wireless event monitoring system, is presented in FIG. 4. Thenode comprises one or more antennas 1000, a radio transceiver 2000, aprocessor 3000, a clock source 4000 and a power supply 5000. In case thenode comprises more than one antenna, the node usually comprises also anantenna selecting switch 6000. The node might comprise other functions7000 too, depending on the type of the node. For example monitoringdevices 200 comprise sensing functions, signaling devices 300 comprisesignaling functions, and human interfacing devices 400 comprise humaninterfacing functions.

In wireless alarm systems the central nodes are typically fitted withtwo antennas and employ receive antenna diversity on the uplink andtransmit antenna diversity on the downlink. The monitoring devices,which are typically much lower-cost devices, may be fitted with oneantenna only.

The simplified functional blocks shown in FIG. 4 do not necessarilyreflect the actual components that are used in constructing the node.For example, the implementation of the radio transceiver might requireauxiliary components such as a low noise amplifier (LNA), poweramplifier, frequency source and SAW filter. The processing function, asanother example, can be implemented by a single processor or by severalprocessors, for example, one processor to handle the communicationrelated tasks and another processor(s) to handle other function(s).Alternatively, the radio transceiver and the processing function may beintegrated into one component, such as a Texas Instruments CC1110System-on-Chip (SoC) device. The clock source, for example, can beimplemented as an internal function of the processor or as a separatecomponent that is external to the processor.

Wireless event monitoring systems commonly operate over unlicensedfrequency bands. Some examples of unlicensed bands are the 433.29 to434.79 MHz band, the 868 to 870 MHz band, the 902 to 928 MHz band, the2.4 to 2.5 GHz band and the 5.725 to 5.875 GHz band. For wirelesscommunication systems that operate in unlicensed frequency bands, it isadvantageous to utilize a spread-spectrum scheme, such asfrequency-hopping or direct sequence, in order to reduce the probabilityof co-channel interference with other systems operating in the sameband. In some regulatory domains, spread-spectrum is mandatory for thisvery reason. An embodiment of the present invention addresses systemsemploying frequency-hopping schemes, and more specifically, systemsemploying slow frequency-hopping scheme. The term “slow frequencyhopping” means that the frequency-hopping rate is substantially lowerthan the communication symbol rate. In the description below, the term“frequency hopping” always refers to slow frequency hopping.

For a synchronized wireless communication system that employs frequencyhopping, it is advantageous to utilize a synchronized frequency-hoppingscheme. Synchronized frequency-hopping means that the radio channel usedfor communication at a given moment in time is a deterministic functionof the system clock, and thus it is known to all synchronized nodes inthe system. This function is referred to in the present document as thesystem frequency-hopping function. The range of the systemfrequency-hopping function might be the entire set of radio channelsavailable for the system, or a subset of that set. The advantage of asynchronized frequency-hopping scheme is that the receiver knows inadvance on which radio channel a potential message might be expected,and tunes to this radio channel.

An alternative to a synchronized frequency hopping is non-synchronizedfrequency hopping, wherein the frequency-hopping function used by apotential transmitter is not known to the receiver, for example sincethey do not share the same clock. An inherent difficulty withnon-synchronized frequency hopping is that the receiver has no priorknowledge of the radio channel used by the transmitter. One method tosolve this difficulty is by receiver fast frequency scan. According tothis method, the transmitted message begins with a long preamble, andthe receiver continuously scans all the potential radio channels. Ateach radio channel, the receiver stays for a time-period sufficient todetect the existence of a valid preamble. If a valid preamble isdetected on a given radio channel, the receiver stays at that channeland detects the rest of the message. Otherwise the receive switches tothe next radio channel.

One disadvantage of non-synchronized frequency hopping, compared tosynchronized frequency hopping, is that the transmitted messages arenecessarily longer, due to the long preamble, and the currentconsumption of the transmitting nodes is therefore higher. Anotherdisadvantage is that the longer messages imply higher occupancy of thewireless media and higher collision probability.

For a synchronized communication system employing frequency-hopping andtime-slotted access, it is advantageous to synchronize the frequencyhopping with the slotted access, meaning that the frequency-hoppingfunction is changed at the beginning of a time-slot. This sort ofsynchronization is use in an embodiment of the present invention.

In order to join a synchronized frequency-hopping wireless communicationsystem, the joining node (such as a remote node in the system of FIG. 1that has not yet synchronized with a central node) needs to becomesynchronized with the system. This process is referred to as initialsynchronization. In order to become synchronized, the joining node needsto adjust its local clock to system clock, and to obtain the informationrequired for calculating the system frequency-hopping function. Onemethod for initial synchronization with a synchronized frequency-hoppingwireless communication system is active synchronization with receiverfast frequency scan.

The method of active synchronization with receiver fast frequency scanis illustrated in FIG. 5. According to this method, the joining nodeselects a radio frequency (denoted in the figure by f(m) and a moment intime for transmitting a probe message. The radio channel f(m) isselected from a predetermined set of radio channels, which might be theentire set of radio channels available for the system, or a subset ofthat set. The selection of the radio channel and the transmission momentare performed locally, since the joining node is not yet synchronizedwith the system. The probe message comprises a long preamble, which isused by the receiver for fast frequency scan. The central nodescontinuously scan all the radio channels (denoted by f(1), f(2), . . .in the figure) within the set of channels allocated for non-synchronizedtransmissions, staying in each radio channel for the minimum durationrequired to detect the existence of a valid preamble. If a validpreamble is detected on some given radio channel, the receiver stays onthe given radio channel and detects the entire message. If the messageis successfully detected, the central node transmits a response messageon radio channel f(m), or on another radio channel known to both sides.The response message comprises a time-stamp and other information neededfor synchronization, for example the parameters of the frequency-hoppingfunction. Note that the response message might be a standard ACKmessage, provided that the standard ACK message comprises all theinformation needed for initial synchronization. The response messagemight also be a special ACK message with additional information, or aspecial probe response message.

A potential alternative to active synchronization is passivesynchronization, wherein the central node periodically transmits beaconmessages, and the joining node employs fast frequency scan until abeacon message is detected.

Active synchronization with fast frequency scan is an advantageousmethod for wireless event monitoring systems, compared to passivesynchronization, some of the advantages being a shorter joining processand reduced current consumption. On the other hand, the scheme of activesynchronization with fast frequency scan apparently conflicts with thescheme of time-slotted synchronized frequency hopping, which is anadvantageous scheme for the steady-state operation of the eventmonitoring system. The conflict is caused by the fact that the latterscheme dictates that the receiver reside on one given radio channelduring each time-slot, while the former scheme dictates that thereceiver scan continuously over all radio channels. In order to resolvethis conflict, in an embodiment of the present invention, the scheme oftime-slotted, synchronized frequency-hopping is utilized by thesynchronized remote nodes, the scheme of active synchronization withreceiver fast frequency scan is utilized by the non-synchronized remotenodes, such as joining nodes, and the central node receiver employs bothschemes concurrently.

The method of concurrent employment of the two schemes is illustrated inFIGS. 6( a) through 6(c), wherein FIG. 6( a) illustrates the case of noremote node transmission, FIG. 6( b) illustrates the case oftransmission by a synchronized remote node, and FIG. 6( c) illustratesthe case of transmission by a non-synchronized remote node. Eachtime-slot (TS) comprises a system frequency window (SFW) and a scanningwindow (SCW). When transmitting an uplink messages, a synchronizedremote node always starts transmitting at the beginning of the SFW. Morespecifically, the remote node starts the transmission a short time afterthe beginning of the SFW, in order to compensate for potentialmisalignment between the local clock of the remote node and the systemclock. The uplink message transmitted by the remote node comprises ashort preamble, denoted in the figure as pr, followed by the content ofthe message, denoted in the figure as data. The uplink message istransmitted at the current value of the system frequency-hoppingfunction, denoted in the figure by fs(n)), to indicate that thefrequency is a function of the time-slot number n. The SFW is longenough to accommodate a minimal detectable preamble, plus the tolerancedictated by the potential misalignment of the clock at the remote node.

During the SFW, the receiver of the central node stays at fs(n) andattempts to detect a valid preamble. If a valid preamble is detectedduring the SFW, the receiver of the central node stays at fs(n) anddetects the rest of the message. If the message is detectedsuccessfully, and the message requires acknowledgment or some otherimmediate reply, the central node replies at fs(n), or at anotherfrequency known to both sides. If a valid preamble is not detectedduring the SFW, the receiver of the central node starts a fast frequencyscan over the radio channels assigned for non-synchronizedtransmissions, denoted in the figure as fu(i)). If a valid preamble isdetected at some radio channel, denoted by fu(x), during the SCW, thereceiver stays at fu(x) and attempts to detect the entire message. Ifthe message is successfully detected, and the message requiresacknowledgment or other reply, the central node transmits the reply atfu(x), or at some other radio frequency known to both sides. If a validpreamble is not detected during the SCW, the receiver stops the fastfrequency hopping during the SFW of the next time-slot, and continuesthe fast frequency scan at the SCW of the next time-slot.

One of the advantages of the method of concurrent employment of the twoschemes lies in the fact that the receiver employs the fastfrequency-scanning scheme during the idle periods of the time-slottedscheme. Thus, the employment of one scheme does affect the efficiency ofthe other scheme.

When joining a synchronized wireless communication system, the joiningnode compares the current value of the system clock, as expressed in thetime stamp incorporated in the reply message received from the centralnode, with the current value of it local clock, and aligns its localclock with the system clock. Assuming hypothetically that the systemclock and the local clock have exactly the same frequency, the localclock will theoretically remain identical to the system clock for anunlimited period of time. In practice, however, there is usually aslight difference between the frequencies of the local clocks, whichcauses the difference between their values to increase gradually.Therefore, for a synchronized node to maintain synchronization, the nodeneeds to receive messages comprising time-stamps and to align its localclock according to those time-stamps, wherein the maximum time intervalbetween received time-stamps is inversely proportional to the maximumfrequency difference between the local clocks and to the maximumallowable synchronization error. Consider for example the case in whichthe accuracy of the local clocks is 50 parts per million (ppm) and themaximum acceptable synchronization error is 1 ms. In this case themaximum difference between the frequencies of the system clock and thelocal clock might reach 100 ppm and therefore the maximum intervalbetween received time-stamps needs to be below 10 seconds.

In order to increase the synchronization accuracy, or increase theallowable gap between time-stamped messages, the frequency of the localclock of the remote node may be adjusted relative to that of the systemclock, using a method such as the following: Let the value of the localclock of the remote node when receiving a first time-stamp be t(1), andlet the value of the first time-stamp be x(1). When receiving the firsttime-stamp, the remote node sets its local clock to the value x(1)+d,wherein d is the estimated delay between the creation of the time stampand the reception of the time-stamped message. The remote node alsosaves the value x(1) for future reference. Now let the value of thelocal clock of the remote node when receiving a second time-stamp bet(2), and let the value of the second time-stamp be x(2). When receivingthe second time-stamp, the remote node sets its local clock to the valuex(2)+d and saves the value x(2) for future reference. The remote nodealso calculates the value f=(t(2)−x(2))/(x(1)−x(2)), which is anestimation of the frequency different between the local clock and thesystem clock. The remote node then adjusts the frequency of its localclock by a·f; wherein a is some positive factor, lower than or equal tounity.

The difference in frequency between local clocks is due to long-termreasons, such as the inherent variation between the devices and effectof aging, and to short-term reasons such as temperature variations. Dueto the short-term reasons, frequency adjustment should also be performedregularly, for example at each reception of a time-stamped message.

As seen above, the process of calculating the frequency difference finvolves division, an operation that might be rather complex for theultra-low-power and ultra-low-cost processor of the remote node. In anembodiment described hereinbelow, the division may might be avoided byusing the periodic nature of the status messages.

As explained above, in order to maintain synchronization a remote nodetypically needs to receive regularly time-stamped messages. A commonmethod for ensuring regular reception of time-stamped messages is tohave the central nodes periodically transmit beacon messages that aretime-stamped, and to have the remote nodes receive those beaconmessages. One disadvantage of this method is that when a remote nodefails to receive a beacon message, it must wait for the next beaconmessage, which increases the risk of losing synchronization.

Another disadvantage of this method for a wireless event monitoringsystem is the extra current consumption of the remote node, due to theneed to wake up to receive the beacon messages.

A method of synchronization of a wireless mesh network without usingdedicated beacon messages is described in a white paper entitled:“Technical Overview of Time Synchronized Mesh Protocol (TSMP)” by DustNetworks, 30695 Huntwood Avenue, Hayward, Calif. 94544,USA (documentnumber: 025-0003-01, last revised: Jun. 20, 2006). This method may beused in conjunction with the embodiments of the present invention thatare described hereinabove.

In an embodiment of the present invention, a wireless event monitoringsystem may be synchronized without using dedicated beacon. Thisembodiment uses a method which is based on the regular nature ofperiodic status messages sent by the remote nodes of the eventmonitoring system. According to this method, the ACK messagestransmitted in reply to the periodic status messages comprise time-stampfields, and synchronization is based on those time-stamp fields. Oneadvantage of this method is that no extra current consumption isrequired for the reception of those time-stamped messages. Anotheradvantage of this method is that the ARQ mechanism ensures periodicreception of time-stamped messages, because when the ACK reply fails tobe received by the remote node, the status message is retransmittedseveral times, until an ACK is successfully received. As a result, theprobability of losing synchronization due to failure to receive atime-stamp message is very low. Another advantage is due to the periodicnature of the periodic status messages. Since the interval betweenstatus messages is fixed (for example 100 seconds), the interval betweensuccessive time-stamps is also fixed, and therefore the frequencydifference f=[t(2)−x(2)]/[x(1)−x(2)] can be approximated byf=[t(2)−x(2)]·k, where k is a constant, thus avoiding the divisionoperation mentioned above. In summary, this method results in highaccuracy and good reliability of the synchronization mechanism, alongwith relaxed requirements on the stability of the local clock.

A remote node of a synchronized wireless event monitoring system mightlose its synchronization with the system, for example, due to failure inreceiving messages for a period of time long enough to cause thedifference between the system clock and the local clock to exceed themaximum tolerance. In such cases, the node needs to re-synchronize tothe system, and the process of re-synchronization is similar to that ofinitial synchronization. According to an embodiment of the presentinvention, re-synchronization is performed according to the scheme ofactive synchronization with receiver fast frequency scanning. Accordingto this embodiment, a node that has lost synchronization with the systemtransmits re-synchronization probe messages according to the scheme ofreceiver fast frequency scan, as presented in FIG. 5, and those messagesare received and replied to by the central node according to the schemepresented in FIG. 6( c).

A remote node of a synchronized wireless event monitoring system thathas lost synchronization with the system might fail to re-synchronizewith the system for a long time. One reason for such a situation mightbe a persistent interference source located in the vicinity of a givenremote node. If the given remote node is a monitoring device, it wouldbe advantageous for the node to be able to transmit operative messages,such as event reporting messages and periodic status messages, althoughthe node is not synchronized with the system. According to an embodimentof the present invention, the scheme of receiver fast frequency scan isutilized also by nodes that have lost synchronization in order totransmit operational messages, such as periodic status messages andevent reporting messages. According to this embodiment, a node that haslost synchronization with the system transmits the operational messageaccording to the scheme of receiver fast frequency scan, as presented inFIG. 5, and the message is received and acknowledged by the central nodeaccording to the scheme presented in FIG. 6( c). If the ACK reply forthe operational message is received by the non-synchronized node, thenode uses the time-stamp field in the ACK message to re-synchronize tothe system. Otherwise the non-synchronized node repeats the operationalmessage several times, until an ACK is received or until somere-transmission limit is exceeded. Thus, the operational message servesboth for delivery of information from the remote node to the centralunit, and for re-synchronization.

As explained above, a wireless event monitoring system in which allremote nodes are two-way nodes performs better than a system in whichthe monitoring devices are one-way nodes. Yet, in some situations, itmight turn out to be advantageous to allow for the incorporation of someone-way nodes along with the two-way nodes. According to an embodimentof the present invention, one-way nodes might be incorporated into thesystem, wherein the one-way nodes transmit their operational messagesaccording to the scheme of receiver fast frequency scan, as presented inFIG. 5, and the messages are received by the central node according tothe scheme presented in FIG. 6( c), wherein the ACK is superfluous.

An embodiment of the present invention addresses the power-save schemeemployed for the downlink. According to the power-save scheme, asexplained above, downlink messages are transmitted at determined pointsin time, referred to as downlink windows. An advantage of the power-savemode for a wireless event monitoring system is the great reduction incurrent consumption. A limitation of the power-save mode is that adownlink message can be repeated only at the next downlink window.Therefore, it is desirable to protect the downlink messages from beinginterfered with by uplink messages. This feature is especially importantfor the downlink messages sent to the signaling devices, because thosemessages require high reliability and short latency.

A straightforward method for protecting the downlink messages from beinginterfered with by uplink messages is to identify the uplink time-slotsthat overlap with the downlink windows and to avoid the utilization ofthose time-slots for transmission of uplink messages. The disadvantageof this straightforward method is that it reduces the number ofavailable time-slots for uplink transmission, thus implying an increaseof the collision probability between uplink messages.

An alternative method for protecting the downlink messages from beinginterfered with by uplink messages is provided by an embodiment of thepresent invention. According to this method, the radio channel used fortransmission of the downlink messages is determined not by the functionused for the uplink transmissions, which is referred to as the uplinkfrequency-hopping function, but rather by a different function referredto as the downlink frequency-hopping function.

It would be desirable, as far as is permitted by regulations, to havethe two frequency-hopping functions orthogonal to each other, whereinthe term orthogonal means that the two functions never collide, thusachieving full protection of the downlink messages from being interferedwith by uplink messages. In cases in which regulations do not permitemploying orthogonal frequency hopping functions, for example in FCCregulations, pseudo-orthogonal functions can be utilized. The termpseudo-orthogonal means that each frequency hopping function is adifferent pseudo-random function. Since the two functions are different,the probability of collision is about 1/n, wherein n is the number ofradio channels in the range of the frequency hopping function.

As explained above, the damage caused by failure in receiving a downlinkmessages is higher than the damage caused by failure in receiving anuplink message, because retransmission of an uplink message can takeplace at one of the near time-slots, while retransmission of a downlinkmessage can take place only at the next downlink window. Therefore, inorder to protect the downlink messages, these messages have priorityover the uplink messages. This priority is achieved by implementing thefollowing algorithms at the transmitter side and at the receiver side.

At the transmitter side, the algorithm for transmitting of downlinkmessages is as follows: At each downlink window, if the central nodedoes not have any outstanding downlink message, the central nodereceiver carries on with the reception of potential uplink messages,using the uplink frequency hopping function for reception of potentialmessages from synchronized nodes, and, if applicable, employing fastfrequency scan for reception of potential messages from un-synchronizednodes. On the other hand, if the central node does have an outstandingdownlink message, the central node transmits the message in the downlinkwindow, using the downlink frequency hopping function. Thus,transmission of a downlink message has priority over reception of apotential uplink message. Yet, since downlink messages are infrequent,this priority is applied infrequently and has only a minor effect on thereception of uplink messages.

At the receiver side, the algorithm for reception of downlink messagesis as follows: In each downlink window applicable to a given remotenode, the node tunes its receiver to the current value of the downlinkfrequency hopping function and attempts to receive a potential downlinkmessage. Reception of downlink messages has priority over transmissionof uplink messages, meaning that a given remote node never transmits anuplink message during a downlink window applicable for the given node.Yet, a remote node can transmit uplink messages during downlink messagesapplicable to other remote nodes. In order to appreciate the advantageof this arrangement, consider an example of a wireless alarm system, inwhich the rate of the downlink windows of the monitoring devices is muchlower than that of the signaling devices, for example once per minutefor the monitoring devices versus once per second for the signalingdevices. In this example, the monitoring devices are not prevented fromtransmitting uplink messages during the downlink windows of thesignaling devices, and the probability that an uplink messagetransmitted during those downlink windows will be lost due to aconcurrent downlink message is low, since downlink messages aretypically infrequent.

Downlink messages are transmitted to the remote nodes either directly orvia one or more repeaters, which implies that there should be amechanism for transmitting downlink messages from a central device to arepeater, wherein the term “central device” refers to the central nodeand to any of the other repeaters. This mechanism can be implementedaccording to the following two methods:

The first method for transmitting downlink messages from a centraldevice to a repeater is similar to the method for transmitting downlinkmessages from a central device to a remote node. The algorithm employedby the central node is identical to the algorithm for transmittingdownlink messages to remote nodes. (Actually, a message transmitted bythe central node might be simultaneously addressed to more than oneremote node and/or repeater.) The algorithm employed by the repeater issimilar to the algorithm employed by a remote node. At each downlinkwindow, the repeater stops its activity in receiving potential uplinkmessages, sets its receiver to the current value of the downlinkfrequency hopping function, and attempts to receive a potential downlinkmessage.

The second method for transmitting downlink messages from a central nodeto a repeater is similar to the method for transmitting uplink messagesfrom a remote node to a central node. The algorithm employed by thereceiving repeater is identical to the algorithm for receiving uplinkmessages. Actually, the repeater employs the same scheme for receivingboth uplink messages from remote nodes and downlink messages from othercentral nodes. The algorithm employed by the transmitting central nodeis similar to the algorithm employed by a remote node for transmittingan uplink message. When transmitting a downlink message to a repeater,the central node is necessarily unable to receive uplink messages, butsince downlink messages are infrequent, this fact has minor effect onthe probability of missing an uplink message.

One advantages of the second method is the better reliability andshorter latency of the hop between the central node and the repeater,because the time gap between retransmissions is much shorter.

Another advantage of the second method is that it avoids the maindrawback of the first method, which is the fact that during the downlinkwindows the repeater has to switch to a different radio channel and istherefore not able to receive potential uplink messages.

In a wireless communication link that is subject to multipathpropagation, the propagation channel between the transmitting antennaand the receiving antenna can vary significantly as a result ofrelatively small displacements of the antenna. Furthermore, thepropagation channel between the transmitting antenna and the receivingantenna might also vary significantly as a result of the relativeorientation of the two antennas, due to polarization. Therefore, it is acommon practice in wireless communication systems to utilize antennadiversity, by fitting the transmitter or the receiver, or both, withmore than one antenna (usually two). The antennas are located at somedistance from one another, and might also have different orientations(usually perpendicular to one another). A simple and common method forantenna diversity is switched antenna diversity, which can be employedby the transmitter and/or by the receiver. According to this method,when transmitting or receiving via a given antenna results in asignificantly better propagation channel than via the other antenna, thebetter antenna is selected for transmission or reception, respectively.

FIG. 7 is a timing diagram that schematically illustrates a method ofintra-message antenna diversity that may be implemented by a centralnode of a wireless event monitoring system, in accordance with anembodiment of the present invention. In this example, the central nodeuses intra-message antenna diversity for the reception of a synchronizeduplink messages, using the method illustrated in FIGS. 6( a) and 6(b).According to FIG. 7, the SFW comprises three sub-windows, denoted in theFigure as SW(1), SW(2) and SW(3). During SW(1) the central node receiveris switched to the first antenna and measures some quality parameter ofthe signal received via the first antenna, such as the received signallevel. During SW(2) the central node receiver performs the sameoperation for the second antenna. During SW(3) the central node receiveris switched to the antenna with the higher quality parameter, andattempts to detect a valid preamble. The subsequent operation of thecentral node receiver depends on the result of the preamble detectionprocess, as illustrated in FIGS. 6( a) and 6(b). In a similar manner,the method of intra-message antenna diversity can also be applied forthe reception of non-synchronized uplink messages in accordance withFIGS. 6( a) and 6(c), wherein each of the time-periods denoted in theFigures by fu(i) comprises sub-windows SW(1), SW(2) and SW(3) with thesame functionality.

The disadvantages of this method of intra-message antenna diversity isthe excessive length of the preamble that it requires. In this methodthe preamble needs to accommodate SW(1) and SW(2) in addition to SW(3),which is required regardless of the antenna diversity function. Thisdisadvantage of the of intra-message antenna diversity method isespecially significant for a wireless event monitoring system, since forsuch systems, the longer preamble means (a) higher current consumptionat the remote nodes and (b) longer time-slots, which implies fewertime-slots per frame. An alternative embodiment, described hereinbelow,provides a method of transmitter-selected receiver antenna diversitythat requires no excess preamble length.

FIG. 8 is a timing diagram that schematically illustrates a method oftransmitter-selected receiver antenna diversity that operates inaccordance with the uplink time-slotted scheme of FIG. 2. According tothis embodiment, each time-slot (TS) within a frame is associated with agiven antenna, and the antenna associated with a given time-slot isutilized by the receiver of the central node to receive synchronizeduplink messages transmitted during the given time-slot. The associationbetween time-slot number and antenna number is a fixed function,referred to as the antenna switching function, that is also known to theremote nodes. In the example presented FIG. 8, the antenna switchingfunction associates the odd time-slots with the first antenna (denotedin the Figure as ANT1) and the even time-slots with the second antenna(ANT2). FIG. 8 also shows that the first two time-slots in each frameare reserved for periodic status messages, while the rest of thetime-slots are available for random access.

Since the antenna switching function is known to the remote nodes, eachremote node that has an outstanding uplink message is able to select theantenna of the central node that will receive the message by selectingthe time-slot in which the message is transmitted accordingly to theantenna switching function. In the case of a periodic status message,the remote node is able to select one of the two time-slots reserved forperiodic status messages, and in the case of random access, the remotenode is able to select one of the time-slots available for randomaccess. In case of retransmission, the time-slot for retransmission isselected according to the random back-off mechanism and according to theantenna selection mechanism.

In order to select the antenna of the central node for reception of anoutstanding message, each remote node calculates the score of each ofthe antennas of the central node. Any suitable method of calculation maybe used. For example the antenna used for the last successful uplinktransmission might have a high score and the other antenna(s) a lowscore. Calculating the score can also involve more elaborate statistics,for example, the number of successful receptions via each antenna duringthe last x successful receptions or the last y time units. Calculatingthe score can also depend on link quality parameters associated witheach successful transmission. The link quality parameters can be signalquality parameters measured by the remote node when receiving the ACKreply. A simple, useful and commonly-used signal quality parameter isthe received signal level, which is inversely proportional to thechannel attenuation, assuming a fixed or known transmit signal level.

Alternatively or additionally, the link quality parameters can be signalquality parameters measured by the central node when receiving theuplink message, provided that these parameters are incorporated in theACK reply. For example, the signal quality parameters can comprise thereceived signal level, which is commonly incorporated in the ACK replyin order to facilitate transmission power control. It should be notedthat in bi-directional communication links employing the TDD scheme,which utilize the same radio frequency for both directions, thepropagation channel between the antennas is reciprocal, and thereforesimilar scoring applies to both directions of the link.

Communication systems usually employ ARQ, as described above. Accordingto this method, when a message is not replied by an ACK, the message isretransmitted according to some random back-off mechanism. Whentransmitter-selected receiver antenna diversity is employed, thetime-slot for retransmission is selected according to the randomback-off mechanism and according to the antenna selection mechanism. Thedesign of the antenna selection mechanism for retransmission depends onvarious parameters. For example, if the temporal variations of thepropagation channels are expected to be slow, a suitable policy would beto retransmit several times to the same antenna before switching to theother antenna. The advantage of such a policy is that it minimizes theaverage number of retransmissions. But on the other hand, such a policyincreases the worst-case time for message delivery. Therefore, in casesin which worst-case delivery time must be short, or in cases in whichthe temporal variations of the propagation channel are quick, a moresuitable policy would be to switch to the other antenna at eachretransmission.

Uplink antenna diversity has been discussed above in the context of asingle-antenna transmitter and a multi-antenna receiver, which is thecommon case for event monitoring systems since monitoring devices 200are typically single-antenna nodes. However, cases of uplink antennadiversity between a multi-antenna transmitter and a multi-antennareceiver may also be applicable. For example, signaling device 300 andhuman interface device 400 of a wireless event monitoring system aretypically bigger and more expensive, and may be fitted with more thanone antenna. Another example are repeaters 500 of the wireless eventmonitoring system, which are usually fitted with two antennas.

In case of a multi-antenna uplink transmitter, two methods can be used.The first method is to select one of the two antennas of the uplinktransmitter as the transmitting antenna, and to employ the same methodas in the case of a single-antenna uplink transmitter. The second methodis to select the transmit antenna according to the receive antenna.According to the second method, the transmitter calculates the score ofeach of its antennas with respect to each antenna of the receiver. Whentransmitting a message at a given time, the transmitter selects fortransmission the antenna with the best score with respect to the antennautilized by the receiver at the given time.

An advantage of the method of selecting the transmit antenna accordingto the receive antenna is that the transmitter is usually able to selectthe transmission time regardless of the antenna switching functionutilized by the receiver, because for each receive antenna there isusually at least one transmit antenna such that the propagation channelbetween the transmit and receive antennas is not subject to severedegradation due to multipath or polarization. The freedom in selectingthe transmission time may, in many cases, be valuable. Consider, forexample, a wireless event monitoring system in which the communicationbetween the central unit and the remote nodes is either direct or via atmost one repeater, and in which the central unit and the repeaters aredual-antenna units. In this system it is advantageous to reserve in eachframe one or more time-slots for forwarding periodic status messages bya potential repeater, thus avoiding any interference between forwardedperiodic messages and other uplink messages. For example, the first twotime-slots in each frame can be reserved for transmission of periodicstatus messages, and the next one or two time-slots might be reservedfor forwarding those messages by a potential repeater, when applicable.Now, the number of time-slots reserved for potential forwarding ofperiodic status messages depends on the antenna diversity methodemployed by the repeater. If the first method is employed, twotime-slots need to be reserved, whereas if the second method is used,one time-slot is sufficient.

Antenna diversity is applicable to the downlink messages, too. Forsingle-antenna remote nodes, the central node should transmit themessage multiple times, once via each antenna, and each transmissionshould take place in different downlink window. The mapping between thetransmit antenna and the downlink window is a deterministic function,which is also known to the remote nodes. The wake-up policy employed bythe remote nodes might be (a) to wake up in the downlink windowscorresponding to each transmit antenna or (b) to wake up only in thedownlink window of the best transmit antenna. The first policy can beemployed when the temporal variations of the propagation channels areexpected to be slow, the potential increase in latency can be tolerated,and the current consumption requirements are strict. The second policycan be employed when the temporal variations of the propagation channelsare expected to be quicker, the potential increase in latency cannot betolerated, and the current consumption requirements are more relaxed.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and sub-combinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

The invention claimed is:
 1. A method for communication, comprising:transmitting a first uplink message from a first remote node to acentral node in a wireless communication system according to a firstfrequency hopping scheme, the first frequency hopping scheme being asynchronized, time-slotted, frequency hopping scheme; transmitting asecond uplink message from a second remote node to the central node inthe wireless communication system according to a second frequencyhopping scheme, the second frequency hopping scheme being anon-synchronized frequency hopping scheme; and concurrently employingboth the first and the second frequency hopping schemes at the centralnode to receive and process both the first and the second uplinkmessages at the central node, said concurrently employing comprisingemploying said second frequency hopping scheme during idle periods insaid first frequency hopping scheme, the second remote node transmittingthe second uplink message using the second frequency hopping scheme inorder to re-synchronize with the central node after having lostsynchronization.
 2. The method according to claim 1, wherein receivingthe second uplink message comprises scanning a receiver of the centralnode over the frequencies used in the second frequency hopping scheme inorder to detect the second uplink message.
 3. The method according toclaim 2, wherein the second frequency hopping scheme is used by remotenodes in the wireless communication system that are not synchronizedwith the central node.
 4. The method according to claim 1, andcomprising synchronizing the first remote node with the central nodeprior to transmission of the first uplink message.
 5. The methodaccording to claim 4, wherein receiving the second uplink messagecomprises scanning a receiver of the central node over the frequenciesused in the second frequency hopping scheme in order to detect thesecond uplink message.
 6. The method according to claim 5, wherein thesecond frequency hopping scheme is used by remote nodes in the wirelesscommunication system that are not synchronized with the central node. 7.The method according to claim 6, wherein the second remote nodetransmits the second uplink message using the second frequency hoppingscheme in order to join the wireless communication system and becomesynchronized with the central node.
 8. The method according to claim 6,wherein the second remote node transmits the second uplink message usingthe second frequency hopping scheme in order to deliver an operationalmessage to the central node.
 9. The method according to claim 6, whereinthe second frequency hopping scheme is used by remote nodes in thewireless communication system that comprise a one-way radio transmitterfor delivering operational messages.
 10. The method according to claim1, wherein the concurrently employing comprises defining a time-slotthat comprises a system frequency window (SFW) for detecting a preambleto the first uplink message and a scanning window (SCW) for detecting apreamble to the second uplink message.
 11. The method according to claim10, wherein said detecting the preamble to the first uplink messagecomprises tuning the receiver of the central node during the SFW to acurrent frequency value of a system frequency-hopping function, and themethod also comprises, upon receiving a valid preamble, remaining tunedto the current frequency value until the entire first uplink message hasbeen received.
 12. The method according to claim 11, and comprisingdefining an antenna switching function of the central node, the functionspecifying respective time slots in which each first antenna of aplurality of first antennas of the central node is to receive signals,wherein transmitting the first uplink message comprises selecting, foreach first antenna of the central node, a respective favored secondantenna among two or more second antennas of the first remote node fortransmitting signals that will be received at the central node via thefirst antenna, and transmitting the first uplink message in a specifiedtime slot via the favored second antenna with respect to the firstantenna specified by the antenna switching function for the specifiedtime slot.
 13. The method according to claim 12, wherein the firstremote node is configured to operate as a repeater, and whereintransmitting the first uplink message comprises forwarding an uplinkmessage received by the first remote node.
 14. The method according toclaim 10, wherein said detecting the preamble to the second uplinkmessage comprises performing a fast frequency scan during the SCW, andthe method also comprises, upon detecting a valid preamble at a givenfrequency, remaining tuned to the given frequency until the entiresecond uplink message has been received.
 15. The method according toclaim 1, wherein receiving and processing the uplink messages comprisesmonitoring events detected by the remote nodes.
 16. The method accordingto claim 15, and comprising issuing an alarm in response to one or moreof the detected events.
 17. A wireless communication system, comprising:a plurality of remote nodes, which are configured to operate inaccordance with first and second different frequency hopping schemes,the first frequency hopping scheme being a synchronized, time-slotted,frequency hopping scheme and the second frequency hopping scheme being anon-synchronized frequency hopping scheme; and a central node, which isconfigured to concurrently employ both the first and the secondfrequency hopping schemes to receive and process both a first uplinkmessage transmitted by a first remote node in the wireless communicationsystem according to the first frequency hopping scheme and a seconduplink message transmitted by a second remote node in the wirelesscommunication system according to the second frequency hopping scheme,said central node being configured to employ said second frequencyhopping scheme during idle periods in said first frequency hoppingscheme, the remote nodes being configured to transmit the second uplinkmessages using the second frequency hopping scheme in order tore-synchronize with the central node after having lost synchronization.18. The system according to claim 17, wherein the central node isconfigured to receive the second uplink message by scanning over thefrequencies used in the second frequency hopping scheme in order todetect the second uplink message.
 19. The system according to claim 18,wherein the second frequency hopping scheme is used by the remote nodesin the wireless communication system that are not synchronized with thecentral node.
 20. The system according to claim 17, wherein the remotenodes are configured to synchronize themselves with the central nodeprior to transmission of uplink messages in accordance with the firstfrequency hopping scheme.
 21. The system according to claim 20, whereinthe central node is configured to receive the second uplink message byscanning over the frequencies used in the second frequency hoppingscheme in order to detect the second uplink message.
 22. The systemaccording to claim 21, wherein the second frequency hopping scheme isused by the remote nodes in the wireless communication system that arenot synchronized with the central node.
 23. The system according toclaim 22, wherein the remote nodes are configured to transmit the uplinkmessages using the second frequency hopping scheme in order to join thewireless communication system and become synchronized with the centralnode.
 24. The system according to claim 22, wherein the remote nodes areconfigured to transmit the uplink messages using the second frequencyhopping scheme in order to deliver operational messages to the centralnode.
 25. The system according to claim 22, wherein the remote nodes inthe wireless communication system comprise at least one one-way node,which comprises a one-way radio transmitter, and wherein the secondfrequency hopping scheme is used by the at least one one-way node fordelivering operational messages to the central node.
 26. The systemaccording to claim 17, wherein the central node is configured toconcurrently employ both the first and the second frequency hoppingschemes in a time-slot that comprises a system frequency window (SFW)for detecting a preamble to the first uplink message and a scanningwindow (SCW) for detecting a preamble to the second uplink message. 27.The system according to claim 26, wherein the central node is configuredto tune during the SFW to a current frequency value of a systemfrequency-hopping function, and upon receiving a valid preamble, toremain tuned to the current frequency value until the entire firstuplink message has been received.
 28. The system according to claim 27,wherein the central node comprising a plurality of first antennas andimplements a predefined antenna switching function, which specifiesrespective time slots in which each of the first antennas of the centralnode is to receive signals, and wherein at least one of the remote nodescomprises two or more second antennas, and is configured to select, foreach first antenna of the central node, a respective favored secondantenna among the second antennas for transmitting signals that will bereceived at the central node via the first antenna, and to transmit thefirst uplink message to the central node in a specified time slot viathe favored second antenna with respect to the first antenna specifiedby the antenna switching function for the specified time slot.
 29. Thesystem according to claim 28, wherein the at least one of the remotenodes comprises a repeater, and the first uplink message is a messageforwarded by the repeater.
 30. The system according to claim 26, whereinthe central node is configured to perform a fast frequency scan duringthe SCW, and upon detecting a valid preamble at a given frequency, toremain tuned to the given frequency until the entire second uplinkmessage has been received.
 31. The system according to claim 17, whereinthe remote nodes are configured to transmit the uplink messagesresponsively to events detected by the remote nodes.
 32. The systemaccording to claim 31, wherein the central node is configured to issuean alarm in response to one or more of the detected events. 33.Apparatus for wireless communication with a central node in a wirelesscommunication system, comprising: a radio transceiver, which isconfigured to transmit uplink messages to the central node in thewireless communication system; and a processor, which is coupled todrive the radio transceiver to transmit a first uplink message to thecentral node according to a first frequency hopping scheme and totransmit a second uplink message to the central node according to asecond frequency hopping scheme, which is different from the firstfrequency hopping scheme, the first frequency hopping scheme being asynchronized, time-slotted, frequency hopping scheme and the secondfrequency hopping scheme being a non-synchronized frequency hoppingscheme, the processor being configured to transmit the second uplinkmessage to the central node using the second frequency hopping scheme inorder to re-synchronize with the central node after having lostsynchronization, the central node comprising a radio transceiver and aprocessor, the processor concurrently employing the first and the secondfrequency hopping schemes, the processor employing said second frequencyhopping scheme during idle periods in said first frequency hoppingscheme.
 34. Apparatus for wireless communication, comprising: a radiotransceiver, which is configured to receive uplink messages transmittedby remote nodes in a wireless communication system in accordance withfirst and second different frequency hopping schemes, the firstfrequency hopping scheme being a synchronized, time-slotted, frequencyhopping scheme and the second frequency hopping scheme being anon-synchronized frequency hopping scheme; and a processor, which iscoupled to the radio transceiver and configured to concurrently employboth the first and the second frequency hopping schemes to process botha first uplink message transmitted by a first remote node in thewireless communication system according to the first frequency hoppingscheme and a second uplink message transmitted by a second remote nodein the wireless communication system according to the second frequencyhopping scheme, the processor being configured to employ said secondfrequency hopping scheme during idle periods in said first frequencyhopping scheme, the processor being configured to process the seconduplink message using the second frequency hopping scheme in order tore-synchronize the second remote node after having lost synchronization.